Friction members used as disc pads for disc brakes and related applications in equipment such as automobiles are typically manufactured by bonding a friction material preform to a backing plate (or shoe) that has been coated with adhesive, while molding and curing the preform within a mold, and subsequently post-curing (heat treating), grit blasting, painting, baking and grinding the assembly so as to give a finished brake pad in which the friction material is bonded to the surface of the backing plate. More specifically, and with reference to FIG. 1, there is shown schematically the basic steps used in a typical prior art method of forming a brake pad. A dry blend or wet slurry of friction material is pressed at room temperature. When using a wet slurry, the preform must then be dried, for example, in a forced air oven. This preform is about 1–2 cm thicker than the final desired thickness dimension for the friction lining. During the room temperature press, surface cracks and flaws may be formed, but are generally of little depth. The dry preform and a backing plate are then placed in the die cavity of, for example, a platen press, a rotary hot eject integrally molded apparatus, a book mold or a positive cavity mold to mold the preform to the final desired thickness, to substantially cure the friction material, and to bond the friction lining to the backing plate by the application of heat and pressure to form a brake pad assembly. The backing plate may first be roll-coated with an adhesive, for example, or the backing plate may be supplied pre-coated with an adhesive to facilitate bonding of the friction lining to the backing plate. The friction material is then post-cured to cure any remaining uncured resin.
During molding and curing, additional surface flaws and cracks may be created due to the temperature differential that occurs between the outer portion of the lining and the inner portion of the lining. A brittle layer (or skin) tends to form initially on the outside, while the inner portion of the lining remains uncured. To minimize the flaws and cracks created by the differential heating, the molding step is generally performed by several repetitions of applying pressure then out-gassing to allow the pad to breathe. The outer brittle layer may nonetheless develop hairline cracks during the later pressing steps.
A brake pad must be scrapped if it contains a flaw that will result in failure of the lining during use. While internal defects and deep flaws and cracks may produce catastrophic failure, minor surface cracks and flaws of little depth will not likely result in catastrophic failure and need not result in scrapping of the part. However, an observer may not be able to ascertain whether the crack or flaw is deep or a mere surface imperfection. Thus, brake pads have typically required painting (also referred to as “coating”) after post-curing because painting hides or reduces minor surface molding imperfections that may be mistaken for deep cracks, such that unnecessary scrapping of the brake pads is reduced. Painting may be accomplished by spray painting, dip coating or powder coating. Spray coatings use a solvent, which is undesirable. Dip coatings apply uneven coatings that create dimensional tolerance problems for the brake pad. Both spray and dip coatings also require a drying step. Powder coating is currently the predominant method for painting the brake pads, which requires that a conductive material be sprayed on the pad first, then dried, to put a charge on the pad, followed by application of the powder, as shown in FIG. 1. The brake pad is then baked to form the coating. This process requires numerous steps, but is solvent-free and capable of providing a uniform coating.
In addition to reducing the appearance of the surface imperfections on the lining such that unnecessary scrapping of parts is reduced, the brake pads require painting after post-curing for several other reasons. One reason is cosmetic, in that the coating typically contains carbon black, or other pigment, and therefore provides a uniform appearance to the friction lining and backing plate. Other reasons for painting include sealing the porous outer surface of the lining to reduce water absorption and to improve edge code stencil visibility.
As stated, the painting is intended to hide or reduce minor surface imperfections. However, powder, spray and dip paint coatings cannot be applied thick enough to completely fill in and smooth these surface imperfections without exceeding the permitted thickness dimensions for the brake pad. In addition, the paints typically include a majority of epoxy or phenolic resin with about 10–15% carbon black and low filler content. These paints produce a thin, shiny coating that is not very effective in filling in and sealing surface imperfections, but rather, often tends to highlight or accentuate the surface imperfections, thereby potentially increasing scrap rates. Thus, current painting methods for disc brake pads include significant capital investment for the painting equipment, particularly for powder coatings, and involves a significant time expenditure to effect the painting, only to have many parts scrapped because the painting is insufficient at hiding and reducing minor surface imperfections.
There is thus a need for a method of painting (coating) brake pads that is effective in filling in and sealing minor surface imperfections to reduce scrap rate in a manner that is time and cost effective.